Sound recording system



April 11, 1939.

G. L. DIMMICK SOUND RECORDING SYSTEM Filed April 20, 1957 tive lm or thelike.

Patented Apr. 11, 1939 SOUND RECORDING SYSTEM Glenn L. Dimmick, Erlton,N. J., assigner to Radio Corporation of America, a corporation of Dela-Application April 20, 1937, Serial No. 137,905

` a claims.

This invention relates to sound recording systems wherein a light beammodulated in accordance with the audio frequency impulses to be recordedis applied to the surface of a light-sensi- It has for its principalobject the provision of an improved apparatus and method of operationwhereby the background noise incident to the reproduction of soun-d fromsuch a record is minimized or avoided.

More particularly, the invention relates to the production of what isknown in the art as a Class A push-pull sound record. Such a recordconsists of a pair of similar sound tracks which are spaced from oneanother and are displaced in phase by degrees. This type of record isbroadly covered by Patent 1,833,373 granted to R. J. Pomeroy on November24, 1931, and assigned to the same assignee as the present application.A method of producing this type of record by means of a galvanometertype of recorder is disclosed by Patent 1,879,423 granted to J. Neale onSeptember 27, 1932, which is similarly assigned. As disclosed in mycopending application Serial No. 47,968, filed November 2, 1935, forRecording of sound, this record may also be produced by means of alight-beam having the form oi' a parallelogram offset to space its endsfrom one another along the length dimension of a light slit across whichit is vibrated in response to the impulses to be recorded. None of theaforesaid inventions, however, involve the provision of means forminimizing the reproduced background noise which results from thetransmission oi light through the unused transparent area of the soundrecord or track and becomes especially objectionable in the case ofreproduced sound of low volume.

It has been proposed to minimize such background noise in various ways,such as by applying to a recording galvanometer winding or the operatingwinding of a shutter a bias potential which varies as the envelope orvolume of the recorded impulses. The present invention in- Volves theuse of a pair of shutters which are operated in accordance with thevolume of the recorded sound to vary the sizes of a pair of oppositelykdisposed trapezoidal light beams which are vibrated transversely of alight slit in accordance with the impulses to be recorded. Thus, whenthe volume of impulses is low, the size of the trapezoidal recordingbeams is relatively small, and vice versa.

The sectional areas of the trapezoidal beams are preferably in the formof a parallelogram on which a triangle is superimposed, the beams be-(ol. 17g-100.3)

ing spaced apart to prevent mixing of the sound tracks and theparallelogram base extensions serving to prevent the light beams fromentirely leaving the slit in case ofovershooting. Other- Wise stated,the light beams are in the form of 5 the displaced halves of adiagonally divided parallelogram.

With respect to the type of shutter utilized, the present invention issimilar to that disclosed by a copending application of Judd O. Baker etal., 10 Serial No. 82,596, filed November 29, 1936, forPhoto-phonographic apparatus and assigned to the same assignee as thepresent application. The present invention, however, differs from thatof the aforesaid Baker et al. application in that 16 it utilizesdifferently shaped light beams and results in an improved and altogetherdifferent type of sound record.

The invention will be better understood from the following descriptionwhen considered in con- 20 nection with the accompanying drawing and itsscope is indicated by the appended claims.

Referring to the drawing Figure 1 is a diagrammatic illustration of arecording system embodying the invention,

Figure 2 illustrates a detail of the system of Fig. l, and

Fig. 3 illustrates the type of sound record produced by the system ofFig. 1.

The system of Fig. 1 includes a sound pick-up 30 device, such as amicrophone I0, from which the impulses to be recorded are suppliedthrough amplifiers II and I2 to the operating coil I3 of a recordinggalvanometer I4 of any suitable type and from which a potentialdependent on the in- 35 tensity or volume of the recorded impulses isapplied through the amplifier II and a rectifier and filter I5 to ashutter operating windin-g I6.

From a light source shown as a lamp l1, light is transmitted through alens I8, oppositely dis- 40 posed trapezoidal apertures I9 and 20 andlenses 2| and 22 to the galvanometer reflector or mirror 23. Light isreflected from the mirror 23 through the lens 22, the light slit 24 of amask 25 and lenses 26 and 21 to the surface of a light 45 sensitivestrip 28 which may be a motion picture film arranged to be moved incontact with a rotatable recording drum 29.

Under these conditions, there is produced at the mask 25 trapezoidallight spots 30 and 3| 50 which are images of the apertures I9 and 20 andwhich may be vibrated transversely of the slit 24 by means of thegalvanometer I4 to produce two similar sound records 32 and 33 (Fig. 3)displaced in phase by 180 degrees. It will be noted that 55 theapertures I9 and 20 are so disposed with respect to one another thattheir images and 3i are slightly displaced from one another along thelength dimension of the slit 24. This spacing of the images isadvantageous in that it prevents overshooting of either of the soundtracks into the path of the other sound track. The parallelogram shapedbase portions of the beams, of course, ensure that high amplitudevibrations do not remove the beam completely from the slit, thusproducing clicks in the reproduced sound.

Coopera-tively associated with the light apertures i9 and 20 .are a pairof shutters 31% and 35 which are pivotedV to supports and 36 by means ofresilient members or springs ''i and 38 and are operated through thesolenoid i6, and a member 39, which is iixedto. the spring 33 at itsupper end, is biased against the pull oi the solenoid i6 by a spring andmay be suitably pivoted at its lower end.

As Will be apparent without detailed explanation, the shutters 36 and 35are moved in opposite directions to cover more or less of thecorresponding apertures i9 and 2B depending on the volume of therecorded sound. Thus, if the volume is low, the vibrational amplitude ofthe images 3] and is relatively short and the shutters 34 and 35 aremoved relatively close together so that a minimum area of sound track isexposed. As the volume of the recorded sound increases, the shutters aremoved further apart to avoid clipping of the record peaks producing inthe positive record of Fig. 3 the shaded areas M and 42. As in previousmethods of noiseless recording by means of a shutter, a minimum oftransparent record area is thus maintained throughout the entire volumerange of the recorded sound.

I claim as my invention:

l. In an impulse recorder including a light slit, means for producing alight beam and means for Vibrating said beam across said slit inaccordance with impulses to be recorded, the combination of meansforming a pair of oppositely disposed trapezoidal apertures .arranged todene the shape of a pair of such beams, and shutter means operable tovary the size of said apertures in accordance with the volume of saidimpulses.

2. In ,an impulse recorder including a light slit, means for producing alight beam and means for vibrating said beam across said slit inaccordance with impulses to be recorded, the combination of meansforming a pair of oppositely disposed trapezoidal apertures arranged todefine the shape of a pair of such beams, and a pair of shuttersoperable in opposite directions to vary the size of' said apertures inaccordance with the volume of said impulses.

3. In an impulse recorder including a light slit, means for producing alight beam and means for vibrating said beam across said slit inaccordance with impulses to be recorded, the combination of meansforming a pair of spaced apart and oppositely disposed trapezoidalapertures arranged to define the shape of a pair of such beams, andshutter means operable to vary the size of said apertures in accordancewith the volume of said impulses.

GLENN L. DIMM'ICK.

